In a conventional art as shown in FIG. 6, it has been known a conventional positioning structure in which a key member 2 is fitted into a key groove 4 wherein the key member (convex portion) 2 extending toward a center of the rotor is formed at an inner wall of the rotor 1 and the key groove (concave portion) 4 extending toward a center of the shaft is formed. (See FIG. 2 of the Patent document 1)
As an improvement of the conventional art, a structure as shown in FIG. 7 in which stress damping grooves 5, 6 is provided at the both sides of the key member 2, respectively (See FIG. 4 of the Patent document 2) and a structure as shown in FIG. 8 in which concave portions 7, 8 is provided at the both sides of the key member 2, respectively (See FIG. 6 of the Patent document 3) have been known.
In case of such a positioning structure, if an electric motor is a PM motor, it is necessary to determine a position of each a magnet embedded at a rotor core and a position of a revolution sensor connected to a shaft so as to control driving force produced by the electric motor.